As a few on SS have begun using aluminum screen frames, I started looking at this myself. What I found is as of the beginning of September, most local stores have either sold out or stopped selling screen frames as they consider them a seasonal item. Kinda silly when you think about it. This is exactly the time of year people are turning off the AC and opening the windows and we need screens to keep out the mosquitos. So I went looking for another source for screen frames.

What I found was an online company that only sells screens and screen parts. They have a large selection of screen materials, and they also carry solar screen fabrics. The screen frames sold locally tend to be lightweight 5/16"x ¾" frame parts. But these guys also carry 7/16" x 1" frames. The wider frames would resist bowing when you stretch your screen material. But the best thing I found is they carry heavy duty screen frames that are 20% thicker than what you can buy locally! These options are ideal for someone making extra large screens. Oh, and you can buy frames pieces as long as 8'. The 8' pieces can be cut in half to use as cross members. Otherwise, with 7' pieces, each cross 4' piece requires an entire 7' piece, with 3' being wasted!

You do have to purchase a minimum of 8 frame pieces, but with longer and stronger pieces, you should have less cut waste and need little or no extra reinforcement. And the longer pieces may incur higher shipping costs.

A wood frame will always be cheap, but tend to be thick, heavy, and warp, which could be an issue if you are trying to fit everything into thinner collector box. The smallest piece of wood you would likely use and still stretch a frame tight, would be roughly 1"x2". And at that size you will need to use premium, knot-free pieces and yet still have a good chance to warp, twist, and bow. Thicker/wider framing also means less screen area for the solar collection material, in this case, screen. Warping or bowing may not be an issue for a standard 2-screen absorber, but warping would be detrimental for collector such as that in a ZP, where screen gap spacing is critical.

All things considered wood is a good choice for most designs, especially if cost is the primary factor.

Thanks Greg. For general info how much would a complete 4'x8' aluminum screen frame cost (excluding the screen)__________________Both temperature rise and airflow are integral to comparing hot air collectors

A 4x8 frame needs 24 LF of heavy duty aluminum frame @ $.33/LF for roughly $8. Add in $4 for the spline and corners and for about $12, you can have a premium aluminum screen frame.

A premium wood frame, 24 LF @$.20/LF would cost you about $4.8. You will likely have to add some metal braces at the corners as well as a pack of staples for another $4. So a premium wood frame might cost you about $9-$10, depending on the size of the braces.

So a premium aluminum frame would cost roughly 20-25% more than the premium wood frame.

As a side note to aluminum frames, I insert 2" long wood blocks inside the frames at assembly wherever I have a thru hole for mounting. This allows me to tighten frames to whatever area of a collector that is needed with out denting the frames. I use very dry hardwood (dried out oak pallet wood) and saw them to a slip fit inside the aluminum frame (+- .005"). Dimensions can be adjusted with a belt sander.

I also use 7/8" long, 8-32 aluminum standoffs (from Ebay) to space screen frames for 4' x 4' screen units. I use 2 frames match drilled to make a ZP insert which rides on foam ledge in a collector. It's also good to use mullions in two directions to hold screen dimensions which are mounted outside the air stream.

I have no doubt you can stretch screens tight enough for a ZP. ZP screens really only have to be tight enough to resist sagging, which would affect the screen gap. Krautman did it with the flimsy screen frames sold locally. Admittedly, his frames were limp noodles that had no stability whatsoever. They felt like they could collapse if you picked them up wrong. But his screens didn’t sag but a ¼” laying flat on the work table. He stretched his screen without any help. As long as the stretched screen surface doesn't sag you should be fine. A little help from an extra hand or cleats, and his screens wouldn’t sag a bit. Take a look at the window screens on your house. Vertically, they probably have no sag, but they may have become a little loose over the years. Collectors like yours that are upright, would be less likely to sag. Whereas my collector tilted back at a 60˚ angle, so gravity may work against it staying tight.

Not only can you get upgrade screen frames made with 20% thicker material, but also wider 1”w v. ¾”, and thicker 7/16” v. 5/16”, which will help against bowing. Remember too, that when I stretched my screens extremely tight, the wood frame 1x2’s were being held by cleats on the tabletop. Surely you could do the same with aluminum frames. And as Don did with his screen frames, attaching them together adds strength and stability.

As far as keeping the screen and spline in the frame groove, Krautman put a few dabs of adhesive to hold the spline in place. A few light peens along the groove would do the same. Or, cover the groove with fool tape like Don did at the corners or maybe strips of flashing covering the spline groove.

One more thing I would suggest to anyone with a collector that is tilted, rather than vertical. If you use a cross brace for your large screen frames, the top screen will be face down. With my wood frame I stapled the screen to the wood cross brace. If you use a metal cross piece, you may want to use some clue to help hold the screen stay tight against the cross brace. The cross brace is also one of the best places to check if your screen surface has any sag.

I haven’t seen any steel screen frames while exploring, but that doesn’t mean they don’t exist. If weight wasn’t an issue, it would surely be stronger and better than any aluminum frame.

Jim-

I like your idea of inserting wood reinforcement inside the screen frame to give you a strong, non-crushable attachment point.

I priced out an order for heavy duty online screen frames from the company linked above. .

To build a 4x16 ZP, with four large frames(4'x7')from a local store(if you can still find them)requires 16 -7' frame pieces, at about $4 each- Total of $64+tax

The heavy duty frames online cost about $5.40 each for 8' pieces Since the pieces can be 8' long, so you only need 12 of them since 8' long pieces can be cut in half for less waste on the 4' cross pieces. 12-8' pieces will total $65+ shipping($13.50 to Minnesota)

So for about the same price, you can purchase heavy duty screen frames. Prices for corners, including aluminum, and spline are as good or better than local. Not a bad deal...

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